The present invention relates in general to an exhaust gas purifying system used with an internal combustion engine of a motor vehicle and more particularly to a throttle valve operating mechanism for an electronically controlled carburetor of the internal combustion engine having at the exhaust system a three-way catalytic converter, the electronically controlled carburetor being operated in response to the state of the exhaust gases entering the three-way catalytic converter.
It is recognized that a three-way catalytic converter comprising catalysts or a catalyst capable of converting harmful exhaust compounds such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NO.sub.x) into harmless compounds performs at its maximum when the air-fuel mixture supplied into the combustion chambers of the engine is kept within a close range of the stoichiometric air-fuel ratio. An electronically controlled carburetor is therefore used in the above-mentioned internal combustion engine system by reason of its increased accuracy in controlling fuel ratio. This kind of a carburetor is so-called "feedback type air-fuel ratio controlling carburetor" which can electronically control the amount of fuel and accordingly the air-fuel mixture to be introduced into an intake manifold of the internal combustion engine by the valving operation of an electric actuator operated in accordance with the composition of the exhaust gases, for example, the concentration of oxygen in the exhaust gases.
Indeed, the above-mentioned electronically controlled carburetor can effectively perform with the three-way catalytic converter when the vehicle is driven normally. However, in a rapid deceleration of the engine due to the rapid release of driver's foot from the accelerator, the electronically controlled carburetor is subjected to the following drawback: owing to a high vacuum in the intake manifold during the deceleration of the engine, a considerable amount of exhaust gases is drawn back into the cylinders of the engine so that the combustible mixture is too lean for normal ignition, besides, the quantity of mixture fed into each cylinder is reduced to a minimum, which again results in misfire.
A number of solutions to these problems has been proposed, one of which is to supply additional or supplementary fuel or mixture into the intake manifold upon engine deceleration for complete combustion in the combustion chamber. One arrangement of this is well shown in U.S. Pat. No. 3,852,391 registered on Dec. 3, 1974 in which a by-pass passage for additional mixture is provided within the carburetor, the passage being opened by a valve sensitive to high vacuum in the intake manifold to allow additional mixture into the intake manifold.
However, this arrangement has not been very effective in respect to the exhaust gas purifying system including the above-mentioned three-way catalytic converter and the electronically controlled carburetor. This is caused by that even if the additional fuel or mixture is supplied into the intake manifold by the operation of such an arrangement upon engine deceleration, the stoichiometric air-fuel ratio is immediately set in the intake manifold by the electronically controlled carburetor thereby causing the above-mentioned drawbacks. Although it is possible to normally combust fuel during the engine deceleration by stopping the electronical control of the carburetor, the harmful compounds in exhaust gases such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NO.sub.x) are not effectively converted by the three-way catalytic converter by the reasons before mentioned.